1. Field of the Invention
The present invention generally relates to testing devices for communications systems, and more particularly to a system for testing voice transmission quality in a cellular communications network.
2. Description of the Prior Art
In the last several years, the telephony industry has seen a dramatic rise in the use of wireless forms of communication, particularly cellular networks. As most cellular subscribers know, however, signal quality in cellular networks is generally much poorer than that found in land-based systems (i.e., copper and fiber optic transmission). Thus, more users are demanding the same quality (audio) in cellular connections as are found in landline connections. Such a parity has yet to be achieved, however, for several reasons. First of all, the causes of signal degradation in a cellular setting are quite different from those in a land-based system. In addition to possible problems in the landline or the public switched telephone network (PSTN), problems can arise at any point in the remaining signal path, including the cellular base station, the radio wave path, and the mobile unit. Moreover, although some systems include means (such as antennas having VSWR and TX alarms) for detecting signal deterioration under certain conditions, there are many causes of reduced signal strength which will not be detected, including water in an antenna, partial lightning damage to an antenna or cable, recently created obstructions (new buildings), a damaged feeder, or a damaged, faulty or waterlogged connector.
The divergence between land-based and cellular transmission quality may also be due in part to the fact that no formal standards have been established for measuring the quality of cellular transmissions. By way of comparison, telephone companies presently use a series of measurements commonly referred to as "105" tests, in order to evaluate signal transmission on telephone trunk lines. The 105 tests have become a de facto standard throughout North America for measuring voice transmission quality. These tests (which are discussed further below in conjunction with FIG. 1) are carried out by one of several available testing systems, such as the START or CAROT systems. START is a trademark of Minnesota Mining and Manufacturing Co. (3M), assignee of the present invention; CAROT is not a trademark, but is an acronym for American Telephone and Telegraph Co.'s centralized automatic reporting on trunks system. These systems utilize certain hardware at each end of the trunk line to be tested, including ROTL's (remote office test line devices), responders and interrogators. In these systems, a controller (such as a personal computer) directs a ROTL to make the appropriate connection to the trunk to be tested, and instructs the ROTL on which of the 105 tests to perform. The test sets at both ends respond to various commands from the controller (via the ROTL), and send the test results back to the controller in an encoded format, such as frequency shift keying (FSK). The controller then generates a report based on the test results.
Most cellular test systems, in contrast, provide little or no capability for objective measurement of voice quality. In general, cellular test systems (not including hardware-specific diagnostic equipment) fall into one of two categories: devices which monitor transmission performance, and those which are used to troubleshoot problems, once found. The most common monitoring devices are system access monitors (SAM's) and "loopback" testers. SAM's (which may be fixed or mobile) scan cellular channels, measuring certain transmission characteristics and recording various system parameters; they may also be provided with geographic referencing. An exemplary SAM is disclosed in U.S. Pat. No. 5,023,900. Although some of these devices do measure signal strength, carrier-to-interference (C/I) ratios, and signal-to-noise ratios, none of them perform even one of the standard 105 tests.
Loopback testers simply check transmission continuity at various sections of the communications path; they generally do not analyze transmission quality at all. Exemplary loopback testers are described in U.S. Pat. Nos. 4,180,708 (checks land-lines between the control station and the base station); 4,415,770 and 4,443,661 (checks wireless transmission between test transmitter and base station); 4,622,438 (checks wireless transmission between subscriber stations and the base station); 4,829,554 (checks land-lines between the control station and the base station, and between given modules within a single base station); and 4,903,323 (checks wireless transmission between a test transmitter and a subscriber station).
Similarly, early troubleshooting devices did not perform any of the 105 tests, although they did measure signal strength and SINAD (a combined measure of the signal-to-noise ratio and distortion). Some of these devices, often referred to as communication system analyzers, also provided various diagnostic tools, such as a wattmeter, a frequency counter, a spectrum analyzer, a voltmeter, and a signal generator. See, e.g., U.S. Pat. Nos. 4,451,712 (for testing base station equipment) and 4,554,410 (for testing a mobile phone). More recently, however, analyzers have been introduced which additionally perform three of the five 105 tests, namely, C-message noise, C-notched noise, and 3-tone slope.
The primary disadvantage of cellular analyzers, even if they are provided with some 105 testing capability, is that they are only used after a problem has been identified, usually by an irate subscriber. Clearly, it would be preferable to perform transmission quality tests continuously to allow early recognition of the problem. Continuous monitoring is all the more desirable considering the possibility of a catastrophic failure in the cellular system. Such failures are relatively common--the mean time between failures (MTBF) for cellular transceivers is about one year; and, even though the MTBF for antennas is about 20 years, since a typical "cellular city" has at least 200 antennas, this works out to an average of 10 antenna failures per year. Another limitation with most analyzers is that they do not perform the tests in the same manner as the customer uses the system. It is thus less likely that the test results will be directly related to how a customer interprets the quality of the audio connection.
One system has, however, overcome many of the foregoing problems. This automated test system for cellular networks, which is very similar to the operation of ROTL's, is depicted in FIG. 1, and is perhaps the closest prior art to the present invention. A controller 1, such as a personal computer (PC), and a test set 2 are located within the mobile telephone switching office (MTSO) 3. Controller 1 is connected to both test set 2 and the switching network 4, whereby controller 1 may direct switch 4 to seize a specific one of the trunk lines 5 leading to a base station 6, and connect that line to test set 2 (lines 5 may be land-based or digital microwave). Controller 1 then issues appropriate commands to test set 2 to dial up the phone number associated with a far-end responder 7 located in the same cell site 8 as base station 6. Once the link is established, controller 1 issues further commands to begin the 105 tests, as well as other tests such as SINAD measurements.
The primary disadvantage in this system is the extreme difficulty of seizing a specific trunk line and passing that line to the test set. Unlike public switch telephone networks (PSTN's), cellular switches have not been designed for such capturing of the trunk lines, so the prior art system of FIG. 1 requires a specialized hardware interface between the switch and the controller, as well as customized software. Moreover, the interface and software are unique for each type of switch (i.e., from different manufacturers). This system is accordingly very expensive, and it still suffers several limitations. First, only one controller at a time may be used on any given switch; this educes the monitoring capability of the system since the switch serves 20-50 cell sites but cannot simultaneously test all of the cells. This limitation is amplified by the fact that there are 15-65 voice channels in each cell. Thus, the prior art system is not truly continuous since different cell sites must share the test set, adversely affecting the system's ability to detect intermittent problems in transmission quality. Secondly, if a fault is detected, this system cannot ascertain whether it is due to a problem with the trunk line or the wireless communication path. Thirdly, the prior art system fails to test the transmission in the same manner as the network is used by a mobile user, since it cannot initiate testing from a mobile station. Finally, while the prior art system can provide regular reports, it does not provide an alarm capability which notifies the network control center immediately upon detection of a potential problem in the transmission system.
Due to the foregoing disadvantages, as well as the cost of this prior art system, most cellular operators still rely on a rather outdated technique for testing voice quality. This technique, known as "calling through" or "drive testing," requires many technicians to drive through the cell sites and manually dial up a special number which emits various audio messages to the technician, who then determines the quality of the transmission. See, e.g., U.S. Pat. No. 5,031,204. Alternatively, the technician may dial up another technician to test both directions. Besides the obvious concern with this subjective analysis, this technique is labor intensive, time consuming, and often incomplete. Moreover, if the cause of poor signal transmission is intermittent, it may not be detected in spite of an apparently thorough search. It would, therefore, be desirable and advantageous to devise an automated, cellular communications test system which would provide improved testing of voice quality transmission, without requiring the difficult seizure of base station trunk lines at the MTSO. The system would preferably provide essentially continuous monitoring of a given cell site, initiated from a mobile station within the cell, with an alarm capability for quick attention to transmission problems. It would also be beneficial to allow determination of the location of any problem as between the base station trunk line and the wireless communication path.